This invention relates to a display device comprising semiconductor devices in which crystalline thin film semiconductor are used, and a method for manufacturing the same. More particularly, the invention relates to an active matrix type liquid crystal display device.
A liquid crystal display device is an image display device having a structure wherein a liquid crystal layer is held between a pair of glass substrates and having a function of modulating visible light passing through the liquid crystal layer by applying an electric field to the liquid crystal layer and thereby causing its optical characteristics to change.
This electric field causing the optical characteristics of the liquid crystal layer to change is formed between pixel electrodes and common electrodes, and desired gradation display can be carried out by the amount of charge entering and leaving each pixel electrode being controlled according to an image signal.
To achieve this, recently the active matrix circuit type display device has become a representative of next-generation displays and has been being researched and developed vigorously.
An active matrix type display device is a device wherein a thin film transistor (TFT) is provided in each of as many as several million pixels arrayed in the form of a matrix, and charges entering and leaving pixel electrodes of the pixels are controlled by means of a switching function of the TFTs.
The pixel TFTs (these multiple pixel TFTs will be referred to collectively as an active matrix circuit) are controlled by circuit TFTs disposed in peripheral driving circuit areas formed at sides of a pixel area. The circuit TFTs constitute different types of circuit such as buffer circuits and shift register circuits according to how they are combined.
That is, an active matrix type display device is a construction wherein pixel TFTs disposed in a matrix in a pixel area and circuit TFTs disposed in peripheral driving circuit areas are all integrated on the same substrate.
However, at present in active matrix type liquid crystal display devices, unevenness occurring and stripe patterns appearing in displays is a problem. In particular, these stripe patterns are extremely harmful to the visual appearance of displayed images.
As a result of carrying out extensive research into display defects appearing as stripe patterns forming when a display device is driven, the present inventors have ascertained that a cause of these defects lies in LDD regions formed in the active layers of the pixel TFTs. Reasons for this are as follows.
In making active matrix type liquid crystal display devices, crystalline silicon films are generally used for the active layers of the thin film transistors. These crystalline silicon films are usually obtained by crystallizing amorphous silicon films.
As the crystallizing means, excimer laser annealing, which has the merit that it is possible to carry out crystallization at a low temperature, is widely used. Crystallization by laser annealing is carried out by irradiating with a laser beam formed into the shape of a line or into a rectangular shape. It is known that generally it is difficult to obtain uniform crystallinity in crystalline silicon films crystallized by laser annealing.
Also, silicon film in a molten state pushed between crystal grains instantaneously growing becomes a solid in a state wherein it has risen just like a wave. When this happens, these parts can be observed as ridges in the surface of the crystalline silicon film obtained.
Thus the crystallinity and the surface condition of a crystalline silicon film obtained by laser annealing vary greatly over the face of the substrate.
When an LDD region is formed, the crystallinity of the crystalline silicon film is disordered by impurity ions being implanted into it and the film temporarily becomes non-crystalline. At this time, due to influences such as those of the above-mentioned differences in crystallinity and differences in the probability of ridges being present, dispersion in the impurity ion concentration arises.
As a result of this, when activation of the impurity ions and recrystallization of the silicon film is carried out by laser annealing, reflecting the above-mentioned dispersion in crystallinity and dispersion in the impurity ion concentration due to the presence of ridges, dispersion arises in the sheet resistance of the LDD regions.
That is, dispersion caused by laser annealing directly has a large affect on dispersion in the sheet resistance of the LDD regions. This dispersion in the sheet resistance of the LDD regions corresponds to dispersion in the on-current of the operating TFTs.
When this dispersion in on-current is large, the accumulation of charge to the pixel electrode is insufficient in some pixels and desired image display becomes impossible. The problem has also arisen that since dispersion in on-current influences the amount of charge accumulated to the pixel electrodes, the pixel electrode holding voltage level changes according to the dispersion in on-current, and it is not possible to obtain desired gradation display.
On the other hand, in circuit TFTs, of which high-speed operation and high output are required, because deterioration due to heat production and hot carriers becomes a serious problem, LDD regions have inevitably been indispensable.
Consequently, when pixel TFTs and circuit TFTs are made TFTs of the same structure, as in Japanese Laid-Open Application 1-289917, LDD regions have always been formed in the pixel TFTs also.
That is, when TFTs having the same structure are used in all the circuits (the active matrix circuit and the peripheral driving circuits), when the requirements of the peripheral driving circuits are considered importance is inevitably attached to deterioration resistance and LDD regions are therefore provided, but in turn this has resulted in dispersion in the on-current of the pixel TFTs and constituted a cause of display defects such as stripe patterns.
There has also been the problem that when LDD regions are disposed in circuit TFTs of buffer circuits, of which a high withstand voltage of about 16V is required, their operating speed falls and the circuit characteristics deteriorate.